Nuclear magnetic resonance (NMR) is a well-known analytic technique that has been used in a number of fields, such as spectroscopy, bio-sensing and medical imaging. In general, an NMR device includes transceiver circuits to transmit signals to a test sample and receive echo signals therefrom. For example, with reference to FIG. 1, the basic components a conventional NMR system 100 include an NMR coil 102 surrounding a sample 104 being analyzed, a magnet 106 for generating a static magnetic field B0 across the sample 104 and the coil 102, a duplexer 108 coupled to the NMR coil 102, and a controller 110 for controlling operation of the various components. Typically, the duplexer 108 includes a transmitter (Tx) portion for delivering RF signals to the NMR coil 102 and a receiver (Rx) portion for receiving echo signals from the sample 104 vis the NMR coil 102.
NMR coil 102 and transceiver 108 are commonly known as an “NMR probe,” which operates with large electromagnets or superconducting permanent magnets in conventional NMR systems. The NMR probe is typically included in an environment having a 50Ω impedance because of a long interconnection required between the probe and NMR instrument.
The RF signals delivered by the duplexer 108 originate with an RF frequency source 115 and a pulse sequence generator 117. A modulator circuit 120 modulates the RF signal from the RF frequency source 115 in accordance with the pulse sequence supplied by the pulse sequence generator 117. The modulated RF signal is amplified by a power amplifier 122.
During NMR measurements, the modulated RF signal having a Larmor frequency ω0 is delivered to the coil 102 via the duplexer 108; the coil 102 generates an RF magnetic field B1 (which is typically orthogonal to the static magnetic field B0) that resonantly excites nuclei spins within the sample 104. After a time duration, Δt, the RF excitation signal is stopped and the controller 110 causes the duplexer 108 to receive the echo signals from the sample 104. Upon stopping the RF excitation, the nuclear spins within the sample 104 precess around the B0-axis at the Larmor frequency ω0. The nuclear spins slowly lose phase coherence via spin-spin interactions, which manifest themselves in a macroscopic average as an exponential relaxation or damping signal in the precession of the net magnetic moment. This NMR signal relaxation can be detected by the coil 102. Because the spin-spin interactions are peculiar to the material of the sample 104 being tested, the characteristic time, commonly referred to as T2, of the relaxation signal is material specific.
The duplexer 108 directs the received echo signals, representing the signal output of the NMR probe, to an amplification block including a pre-amplifier (e.g., a low-noise amplifier 125) and a programmable gain amplifier 127. The signal is ultimately converted to digital form by an analog-to-digital converter (ADC) 130 for processing. But the frequency of the “raw” NMR signal received by the pre-amplifier 125 is too high for the ADC 130, and is therefore “down-converted” through comparison with the signal supplied by the RF frequency source 115. A mixer 135 combines the amplified NMR signal, which oscillates at the Larmor frequency, with the reference signal from the RF frequency source to generate a new signal that oscillates at a lower “relative Larmor frequency.” Following filtering by a low-pass filter 137, the signal varies slowly enough to be handled by the ADC 130 but nonetheless retains the essential frequency characteristics of the received echo signals.
Thus, by measuring the Larmor frequency ω0 described above (e.g., for spectroscopy) and characteristic time T2 (e.g., for relaxometry), NMR techniques can be used as an analytic tool in a number of fields, including but not limited to chemical composition analysis, medical imaging, and bio-sensing.
Significant efforts have been devoted to miniaturizing traditional NMR systems. For example, the entire NMR electronics, including the power amplifier (PA) 122, may be integrated on a single semiconductor device. The numerous advantages of miniaturization include low cost, portability, and the fact that a micro-coil tightly surrounding a small sample increases the signal quality. In addition, reducing the size of the magnet 106 allows use of a much smaller power to excite (or polarize) the sample 104 than in a conventional system.
FIG. 2 depicts a traditional class-D PA 200 implemented in a miniaturized NMR system; the input signal Vs to the PA 200 is typically a square-wave signal having low and high amplitudes between the ground (VSSPA) and the PA source voltage (VDDPA). The input signal is used to close and open switches 202, 204 in an alternating fashion for connecting an output load 206 to either VDDPA or VSSPA. The power transferred to the load 206 depends on the output impedance of the amplifier 200, the input signal amplitude Vs, and the load impedance RL. In a single-ended PA as depicted in FIG. 2, the delivered power can be represented as:
      P    L    =            1      2        ⁢                                                                    Vs                                      2                    ⁢                      R            L                                                (                                          R                Out                            +                              R                L                                      )                    2                    .      
The traditional class-D PA typically delivers a power ranging from watts to kilowatts, but it has a somewhat limited bandwidth (usually much less than 1 MHz). In addition, because the output impedance of the traditional class-D amplifier is fixed, the available power setting is also fixed for the fixed power-supply voltage VDDPA. In other words, the power and output impedance of the amplifier are not separable. Because of this constraint, it is difficult to adjust the power available from the amplifier in order to optimize the excitation parameters (such as magnetization flipping angles and NMR excitation pulse spacing) of an NMR measurement.
Recent developments in class-D amplifier technology have been exploited to generate excitation signals suitable for NMR measurements, particularly in low-field time-domain NMR relaxometry. A number of difficulties, however, exist. For example, in order to make NMR measurements, the PA requires a wide bandwidth (e.g., between 10 MHz and 60 MHz). In addition, impedance matching is important (but harder to achieve in NMR applications compared to classic audio or power applications) for optimizing the power delivery (as shown by the equation above), and PA power levels have to be maintained with accuracy and consistency for repeatable NMR measurements.
Various strategies to address these difficulties have been proposed, generally involving the use of discrete components for the PA and assuming the ON resistance of the switches 202, 204 to be negligible. This allows the output impedance of the PA to be set by an external precision resistor. When integrating the PA and other NMR electronics on a single semiconductor device—resulting in an integrated “switch-mode” power amplifier—the switch devices 202, 204 are typically implemented using MOSFETs whose gates are controlled by an input RF signal having a square wave form with an amplitude of VDDPA VSSPA.
In a typical NMR application, the PA drives a 50Ω load impedance. This means that for the case of a differential class-D PA, each PA driver has an output impedance of 25Ω for an optimal, reflection-free power delivery to the load. The ON resistance of the MOS device, however, varies with manufacturing processes, supply voltage and temperature. In addition, the highly nonlinear behavior for large voltages across the device makes it extremely challenging to implement a basic MOSFET switch having a constant ON resistance of 25Ω.
Accordingly, there is a need for an approach that reduces the variability of the output impedance of an integrated switch-mode power amplifier in order to maintain consistent power levels repeatably during NMR measurements.